1. Field of the Invention
The present invention relates to an optical scanning device, an image forming apparatus and a liquid crystal device driving method, and, in particular, to an optical scanning device or a multi-beam optical scanning device used for a laser writing optical system, an image forming apparatus such as a laser writing optical system, or a laser printer, a digital copier, a laser facsimile machine, a laser plotter, or such.
The present invention also relates to an optical scanning device provided with a liquid crystal device which can modulate phase of a laser beam, to an image forming apparatus, such as a laser writing optical system, or a laser printer, a digital copier, a laser facsimile machine, a laser plotter or such, employing the optical scanning device and a liquid crystal device driving method carried out in the optical scanning device.
2. Description of the Related Art
Recently, performance of an image forming apparatus such as a laser printer, a digital copier or such, has been improved, i.e., image quality is improved, processing speed is increased, and color printing or color copy could be practically carried out. Along with such an improvement of the performance, users are requesting a further higher quality.
For the request for processing speed, to apply a multi-beam manner is effective. However, in this manner, pitch adjustment among a plurality of beams is required. For this purpose, a method of rotating a multi-beam light source about an optical axis, a method of using an optical device prepared for pitch adjustment or such may be applied (for example, see Japanese Laid-open Patent Application No. 9-131920).
For the request for image quality improvement, reduction of a beam spot is required. Various methods have been proposed therefor (for example, see Japanese Laid-open Patent Applications Nos. 3-116112, 5-19190 and 2001-166237).
However, technical problems exist in actually achieving reduction of a beam spot diameter. In particular, it is necessary to consider that an image forming apparatus or an optical scanning device has many heat sources such as a fixing unit, a polygon scanner or such. Accordingly, it is necessary to take account of temperature fluctuation in the operation environment in seeking the reduction of a beam spot diameter.
In the above-mentioned related art of rotating a light source unit about an optical axis, since the light source unit itself is moved in this art, a problem may occur in terms of reliability of the electrical components. Further, in the related art of using an optical device for pitch adjustment, since a high accuracy glass optical device is required, a problem in terms of the required cost may occur.
Further, even if a beam spot diameter is adjusted initially at a high accuracy, a positional shift of the beam spot may occur due to influence of aging such as temperature fluctuation or such.
Therefore, a liquid crystal device driven by an electrical signal has been proposed to carry out the above-mentioned pitch adjustment (for example, see Japanese Laid-open Patent Application No. 6-214177). In this method, a beam pitch is detected by means of a detector, and, based on the detection result, the liquid crystal device is driven. Thereby, it is possible to correct for the aging variation of the beam pitch. The liquid crystal device is advantageous when it is used as a beam pitch adjustment device in that it is driven by a low voltage signal, does not generate heat, does not generate vibration, does not generate noise, is small sized, and also, is light weighted.
This liquid crystal device acting as the pitch adjustment device has a cell structure in which a liquid crystal layer on the order of several through tens of μm is sealed by two glass substrates. Therefore, a central part of the liquid crystal layer having a relatively high thermal expansion coefficient expands when an ambient temperature increases. As a result, a positive power lens function may appear. When the liquid crystal device thus has such a lens function due to increase in the ambient temperature, a beam waist position may shift, and thus, the beam spot diameter may degrade (increase).
In an optical scanning device used in an optical writing part of an image forming apparatus, a rotational speed of a polygon mirror acting as a deflection device may be improved for the purpose of improving a recording speed. However, in this case, a problem may occur in terms of durability, noise or vibration of a polygon motor, a problem in a laser modulation speed or such, and thus, there is a limit in the improvement of the recording speed in this manner. Therefore, as an alternative method, a multi-beam scanning device is proposed by which a plurality of laser beams are used to simultaneously carry out scanning and recording a plurality of scan lines simultaneously.
As a type of a multi-beam light source device emitting a plurality of laser beams used in the multi-beam scanning device, there is a type in which a multi-beam semiconductor laser (for example, a semiconductor array) having a plurality of light emitting points (light emitting channels) enclosed by a single package is used. However, it is difficult to increase the number of the channels in terms of manufacturing process. Also, it is difficult to remove influence of thermal/electrical crosstalk. Further, it is difficult to shorten the oscillation wavelength. As a result, the light source device may become expensive.
On the other hand, many proposals have been made for a light source device in which a single beam semiconductor laser is used as a light source, and, with the use of a beam creation device, a plurality of laser beams are created therefrom, and a multi-beam scanning device uses them. However, in this method, a problem in which an arrangement of beam spots (beam pitch, or scan line separation) on a to-be-scanned surface may change due to environmental fluctuation, aging fluctuation or such may occur. Then, many proposals have been made for solving this problem.
The present applicant made one proposal in Japanese Laid-open Patent Application No. 2003-337293, ‘Optical Scanning Device, Light Source Device and Image Forming Apparatus’. In the art, a ‘liquid crystal device’ driven by an electrical signal is used to correct beam spot arrangement on a to-be-scanned surface. In this art, in order to control the beam spot arrangement on the to-be-scanned surface, the liquid crystal device is used as a ‘light path deflection device’ which is disposed in or immediately subsequent to a light source part, for deflecting a laser beam by a slight angle (several through tens of minutes).
Recently, along with the request for increasing the recording speed of a color image forming apparatus, for example, a so-called ‘four drum tandem type image forming apparatus’ is practically used in a form of a digital copier, a laser printer or such. In this apparatus, four photosensitive drums are disposed along a direction of conveying a recording member (for example, a recording paper) or a direction of an intermediate transfer body; latent images are formed thereon simultaneously with exposure thereof by means of corresponding plurality of scanning optical systems; these latent images are then visualized with the use of developing agents of respective different colors, such as yellow, magenta, cyan and black, or such, by means of a developing device; these visualized images are transferred onto the common recording member or intermediate transfer body in a superposing manner; and thus, a color image is obtained thereon.
This four drum tandem type image forming apparatus is advantageous for carrying out high-speed printing in comparison to a one drum type image forming apparatus, in that a color image can be output in the same speed as that for a monochrome image. However, since this type of machine has the four scanning optical systems for the respective four photosensitive drums, a problem may occur in that miniaturization of the machine may be difficult. Further, another problem may occur for avoiding color drift which may occur when the toner images of the respective colors developed on the respective photosensitive drums are transferred to the recording member or the intermediate transfer body in the superposing manner.
The following causes of generation of the above-mentioned color drift in particular in a sub-scan direction in the four drum tandem type image forming apparatus may be cited for example:
(1) fluctuation of a feeding speed of the photosensitive drum in a circumferential direction (the sub-scan direction);
(2) fluctuation of the feeding speed of the recording member or the intermediate transfer body in the conveyance direction (the sub-scan direction);
(3) positional error among the photosensitive drums;
(4) positional error of beam spots written among the scanning optical systems;
(5) positional error of the above-mentioned items (1) through (4) due to environmental fluctuation, or temperature fluctuation occurring when continuous printing is carried out for example; and
(6) when writing is carried out simultaneously to the respective photosensitive drums in a multi-beam manner, since rotation of a polygon scanner and the photosensitive drum feeding speed are not in synchronization with one another in general, a positional shift for a number of beams may occur in the sub-scan direction.
As a scheme for avoiding the color drift, the following methods are known for example:
(A) As disclosed in Japanese Laid-open Patent Application No. 2001-133718, in an image forming apparatus using a plurality of scanning devices, a position of each entire scanning device (housing) is adjusted with respect to a photosensitive body, and thus, scan lines can be made coincident on the respective photosensitive bodies. However, in this method, a mechanism may become complicated for the adjustment, and also, a time is required for the adjustment. Furthermore, since the housing having a large weight is adjusted in its position, it may be difficult to respond to an aging fluctuation such as temperature fluctuation, and thus, it may be difficult to carry out color drift correction at high accuracy during printing or for possible fluctuation in the operation environment.
(B) As disclosed in Japanese Laid-open Patent Application No. 2001-100127, a galvano mirror is used to control the sub-scan beam position. However, since the galvano mirror has a sensitivity too high to control the sub-scan position, the control may be easily affected by external vibration, and thus, a high surface accuracy (approximately four times that of a transmission surface) is required for maintaining a further satisfactory beam spot diameter.
(C) As a configuration of solving the problem of positional error among the multiple beams, Japanese Laid-open Patent Application No. 10-239939 discloses a color image forming apparatus having a correction device which switches a laser beam to write an image first from among a plurality of laser beams according to a phase relationship between an intermediate transfer reference signal and a line synchronization signal, whereby an image writing start position can be adjusted for each color in a sub-scan direction, and thus, the color drift can be corrected. However, even by applying this method it is not possible to achieve a correction in an amount smaller than one line, and, as a result, a color drift more than 42 μm may occur in a case of writing in 600 dpi (‘dpi’: dots per inch) for example.
Japanese Laid-open Patent Application No. 2002-214579 discloses one example of the prior art for deflecting a laser beam path with the use of a liquid crystal device. In this example relating to an image display apparatus, by applying a voltage to a transparent electrode array formed in an arrangement pitch corresponding to the pixel pitch, an uneven electric field is formed in the liquid crystal layer, and, a light path is deflected by means of refractive index distribution thus generated in the liquid crystal layer.
However, in this case of the deflection device applying the liquid crystal device in the prior art, a voltage is applied between both ends of the transparent electrodes arranged like a stripe with equal separation, and a resistance member is provided such that an electric potential distribution may have a uniform slope (linear characteristic) in the arrangement direction. When the refractive index distribution in the liquid crystal layer is controlled in such a manner that the electric potential distribution in the liquid crystal layer may have a uniform slope, there is a possibility that, even when the electric potential distribution has a uniform slope, the refractive index distribution in the liquid crystal layer may not necessarily have a uniform slope, but may behave to draw a curve which may be approximated by a high-order polynomial. If a laser beam is applied to the liquid crystal layer appearing such a curved refractive index distribution (that is, having a power component), the liquid crystal acts as a lens, and thus, an optical axis direction of an exit beam therefrom changes unexpectedly (that is, the light path is deflected). In addition, also a condition of convergence or divergence of the exit beam may change unexpectedly.